Ice Ages are most likely caused by variations
in the intensity and timing of the heat from the sun which cause
glacial/interglacial cycles. Serbian scientist Milutin Milankovitch
described this in 1938. There are three major components of the
Earth's orbit around the sun that contribute to changes in our
climate. First, the earth's spin on its axis is wobbly, and variate
up to as much as 23.5 on either side of the axis. The tilt in
the Earth's rotation affects the amount of sunlight hitting the
different parts of the world. The greater the tilt the greater
the difference in the seasons. The range of motions in the tilt{left-of-center
to right-of-center and back again} takes 41,000 years to complete.
Asa result of a wobble in the Earth's spin, its position on its
elliptical path changes relative to the time of year. This is
a phenomenon called the precession of equinoxes.

The cycle of equinox precession takes
23,000 years and in the growth of continental ice sheets summer
temperatures are probably more important than winter.Ice Build
Ups-When summers are too cold to allow snow to melt from previous
winters.{summers that occur when the sun is at its farthest point
in Earth's orbit}. When this continues for centuries, ice sheets
begin to form. The orbit of the Earth also begins to change.
At one extreme of this, when the orbit is more circular, each
season recieves the same amount of solar radiation. At the other
extreme, the orbital ellipse is strecthed longer and exaggerates
the difference between the seasons. The eccentricity of Earth's
orbit also proceeds through a long cycle, which lasts for 100,000
years. Major glacial events have coincided with the phases of
the Earth's tilt, precession of equinoxes and eccentricity of
orbit which are lined up to give the northern hemisphere the
least amount of summer solar radiation.

Presently, 10% of the world's total
land area is ocuppied by glaciers with most of them located in
Antarctica or Greenland. Glaciers have been around since the
Ice Age, when ice covered about 32% of the land and 30% of the
oceans. An Ice Age is a period when cool temperatures allow
polar ice to advance into the lower latitudes. During the last
Ice Age on Earth, huge sheets of glacial ice covered all of Canada,
Scandanavia, Scotland, and parts of New York State. Within the
past 750,000 years, scientists have learned that there have been
eight Ice Age cycles with warmer interglacial periods between
them. Right now, the Earth is at the end of an interglacial
period and will experience another Ice Age in a few thousand
years. This is part of the normal cycle of climate change. Glaciers
also serve as signs of possible global climate
change.